Previous work, employing the polymorphonuclear neutrophil reduction in mice and the depression of cell replication in PAM cell cultures, has indicated the association of a virus-like agent(s) with multiple sclerosis. The agent(s) has been shown to replicate in these systems, since cell-free homogenates from either high passage PAM cells or MS-inoculated mice still possess the agent's activity. MS-treated PAM cells will be examined for the presence of a virus-like agent, using radioactive nucleic acid and protein precursors. The agent will be isolated and purified from various culture fractions, utilizing rate zonal and isopycnic centrifugation. Once isolated, the agent will be characterized with regard to its nucleic acid and protein content, and to its electron microscopic appearance Brain-explant cultures from MS-inoculated mice will also be examined for the presence of a virus-like agent(s) employing the procedures described for the PAM cell cultures. MS-infected cells will also be studied for cell surface modifications and nucleic acid and polypeptide alterations. The potential epidemiological relationship of the purified agent to the human disease state will be investigated, using immunologic studies and electron microscopic examination.